1. Field of the Invention
The present invention relates to a multi-color printer and to a method of printing, in particular to a single-pass multi-color printer and to a method of printing multi-color developed toner images on a substrate.
2. Description of the Related Art
To enable printing on a wide variety of recording media, at least one transfer member has to be introduced to transfer a developed toner image from an image-forming station via this transfer member to a recording medium (substrate) where it can be fused. It is a clear benefit that the use of such a transfer member obviates the need for the conditioning of the substrate or at least the conditioning is less demanding.
When a single transfer member is used, it is in direct contact with an image-forming member, which is for example a photoconductive belt or drum, at each image-forming station to receive the image therefrom. As a consequence, the temperature at each contact should be low, preferably below the glass transition temperature of the toner. A higher temperature can lead to (permanent) contamination of the image-forming member which negatively influences the quality of the toner image formed on the image-forming member. However, the transfer and simultaneous fusing (xe2x80x9ctransfusexe2x80x9d) of the toner image to the substrate requires both heat and pressure. The exact temperature of the transfer or transfuse depends on the nature of the transfer member as well as that of the substrate. The temperature has to be close to the softening temperature of the toner in order to guarantee a good transfer. This is typically in the range from 90 to 150xc2x0 C. Therefore, heating means have to be provided to heat at least the portion of the transfer member at the nip between the transfer member and the substrate. Subsequent cooling of the transfer member is required to ensure that the temperature of at least the portion of the transfer member contacting the image-forming members is sufficiently low. To produce such a large temperature change, active cooling means and heating means have to be provided in close proximity to each other. This negatively influences the power consumption, even when making use of some heat exchange. It is clear that all these requirements are difficult to meet simultaneously using a single transfer member, while ensuring a high quality, high resolution single-pass multi-color print.
A high printing speed and multi-color printing facilities are basic requirements for leading edge copy and printing machines, resulting in even more stringent requirements for the transfer processes and the nature of the transfer members. The speed requirements impose a single-pass configuration. In such a configuration, a multiple toner image is formed on the transfer member by electrostatically transferring a plurality of developed toner images in register with each other from a plurality of image-forming stations during a single revolution of the transfer member. The resulting charged multiple toner image is far more difficult to transfer than a single toner image.
In U.S. Pat. No. 5,805,967 (De Bock et al./Xeikon NV), a multi-color single-pass printer is described. The printer includes a plurality of image-forming stations at which developed toner images are formed. Each of these images is electrostatically transferred to an endless primary belt. An isothermal intermediate transfer zone is established by the face-to-face contact between the primary belt and an endless intermediate belt. The intermediate belt is heated downstream of the intermediate transfer zone to heat the image thereon. A final transfer station transfers the heated image from the intermediate belt to a substrate. The intermediate belt is then forcibly cooled before returning to contact the primary belt in the intermediate transfer zone.
In a modification of this printer, shown in FIGS. 16 and 17 of U.S. Pat. No. 5,893,018 (De Bock et al./Xeikon NV), the intermediate transfer zone is formed between first and second guide rollers pressed towards each other to cause extended contact between the primary belt and the intermediate belt. The first guide roller is electrically biased to create an electrical field in the intermediate transfer zone to assist in transferring the image from the primary belt to the intermediate belt. While the extended contact zone enables the transfer of drive from the intermediate transfer belt to the primary belt, it would also result in an excessive transfer of heat to the primary belt, unless the intermediate transfer belt is cooled after transfer of the image to the substrate and before its return to the intermediate transfer zone. Furthermore, the extended contact zone can result in a distortion of the transferred images.
While such a method is capable of producing good quality results, the need to both heat and cool the intermediate belt consumes significant amounts of energy and puts limiting restrictions upon the characteristics of the intermediate belt and its support mechanism.
It is an aim of the invention to provide a single-pass multi-color printer which allows for printing on a wide variety of recording media by making use of two transfer belts between the image-forming members and the substrate. To limit power consumption, for economical reasons as well as for reasons of reliability, simultaneous heating and cooling of the same transfer belt has to be avoided. It is a further aim of the invention to improve the transfer efficiency of the toner image from the image-forming members to the substrate, particularly the transfer from the primary transfer belt to the intermediate transfer belt.
In one embodiment of the invention, the printer comprises a primary image transfer belt receiving a toner image from one or more image transfer stations, an intermediate image transfer belt in nip contact with the primary image transfer belt for image transfer from the primary image transfer belt to the intermediate image transfer belt, and a final transfer station to transfer the toner image from the intermediate image transfer belt to a substrate. In some advantageous embodiments of the invention, the intermediate image transfer belt receives no active cooling.
In another embodiment, the invention comprises a method of printing a single-pass multi-color image comprising forming a toner image on a primary image transfer belt, transferring the toner image from the primary image transfer belt to an intermediate image transfer belt through a nip contact between the primary image transfer belt and the intermediate image transfer belt, and transferring the toner image from the intermediate image transfer belt to a substrate.
Furthermore, a method of minimizing heat transfer between a primary image transfer belt and an intermediate image transfer belt may comprise forming a nip contact between the image transfer belts such that image transfer is accomplished across a temperature gradient without excessive heat transfer from the intermediate transfer belt to the primary image transfer belt.